The mysterious story of the Greenland shark
Although the Arctic appears to be an unlikely habitat for sharks the Greenland shark is found in abundant numbers in the cold Arctic waters. The Greenland shark is an enormous heavy-bodied sleeper shark with a round snout, heavy cylindrical-shaped body and small precaudal fins. The shark has rough skin with strong hook-like cusps. There colour ranges from medium grey to brown with at times small dark bands and spots. When Greenland sharks are born they are around 40 cm long and grow up to between 8 to 14 feet long, with some even reaching 24 feet long. The Greenland shark prefers living in waters along continental shelves and upper slopes up to a depth of 1,200 metres. They are also prefer water temperatures that range from 0.6°C to 12°C. During the Arctic winter the sharks typically move inshore to shallower waters.
Sluggish sharks at the bottom of the ocean
While the Greenland shark is known to be quite a sluggish specie, typically offering little resistance when captured and are easily fished through ice holes, they do eat a variety of species including active fish, seabirds and seals. They have even been known to feed on dead cetaceans and even drowned horses and reindeer. Of interest to scientists is how a possibly luminescent copepod parasite often attaches itself to the cornea of the sharks’ eye to almost lure prey species towards the shark under an almost mutually beneficial relationship. Typically the Greenland shark is hunted for human and sled-dog good and Inuit use their skin for boots and the sharks’ lower dental band as knives. However, before the meat can be consumed by humans or dogs – it is highly poisonous and can lead to stiff movements, hyper-salivation, vomiting, diarrhea, convulsions and even death – it needs to boiled in several changes of water or by burying the meat for several months above the high tide-line so microbial action can chemically modify it.
Greenland sharks’ natural antifreeze
The Greenland shark is highly poisonous due to trimethylamine oxide (TMAO) in the tissue of the sharks’ flesh. TMAO in the tissues of the Greenland shark helps it stabilise their enzymes and structural proteins against the debilitating effects of cold and extreme water pressures. During the summer months the Arctic water temperature may fall below 4°C to even minus 2°C and at that temperature even the most hardiest proteins will cease to function properly without some form of chemical protection. Typically polar fishes manufacture glycoproteins – or antifreeze – while polar sharks store urea and TMAO to prevent ice from crystallising and stabilise proteins. Even with the TMAO the Greenland shark appears to be sluggish as its life processes occur in slow-motion. On average, biochemical processes occur three times more slowly for every 10°C drop in operating temperature. The result is that while the shark may grow into enormous sizes it takes a very long time to grow. In fact, one 2.77 metre-long Greenland shark tagged off western Greenland in 1936 was recaptured 16 years later, measuring 2.86 metres. This indicated an average growth rate of less than 0.56 centimetres per year: at that rate a 6 metre-long Greenland shark would take several centuries to reach that length.
A 400 year-old Greenland shark?
In fact, scientists this year found the possibility that Greenland sharks live to 400 years old or more. Like counting the rings of trees to determine their age scientists analysed Greenland shark eye specimens to determine their age. The transparent tissue in the eye lens is metabolically inactive and that new layers are added throughout the shark’s lifetime. If all the layers are removed according to the researchers at the University of Copenhagen scientists can come to the layer from when the shark was a baby. The scientists examined this tissue of 28 female sharks measuring between 0.8 to 5 metres long, captured in the North Atlantic as bycatch. However, instead of manually counting the layers of tissue the scientists used radiocarbon dating to measure the amounts of a particular carbon isotope absorbed by living tissue in the innermost and so oldest part of the lens.
Determining the Greenland sharks’ average lifespan
Age estimates of the two biggest sharks, which measured 4.9 and 5 metres, placed them at around 335-392 years old. As researchers already knew that female Greenland sharks reproduce once they are 4 metres in length the study was able to estimate that the Greenland sharks are around 156 years old when they start reproducing. The analysis of all the shark tissue indicates that the Greenland shark’s lifespan ranges between 252 and 512 years old, with 390 years the likely average. The scientists did say that even if the sharks lived to the lower range of the estimated lifespan they are still the longest-living vertebrate known to science: Other long living vertebrate species known are the Galapogas turtle that lives to around 150 years, while box turtles live to about 120 years and even swans can reach 100 years old. The challenge now is for scientists to understand more about how this slow-moving shark lives for so long: data of living Greenland sharks very hard to come by as they live at great depths and may be only visible to divers for less than 10 seconds.
Greenland sharks harmless to humans?
While the Greenland shark can be considered ‘monster’ size, the sharks are not Jaws with no confirmed Greenland shark attacks on a live human. However, this does not mean there is not the potential for violent or even deadly encounters with the species. One of the main reasons why there has been no Greenland shark attacks on humans is that they swim at depths that are inhospitable to humans and so there is almost no chance a swimmer or diver of the general population will encounter one in their entire lifetime (we do not either on our Greenland cruises).
Nonetheless, there have been human-Greenland shark encounters in the past with specialist dive teams. The sharks have been observed leaving the bottom to investigate diver activities at the surface with on shark even stalking a team of divers all the way up to the surface after a dive. Scientists speculate that this could be indicative of visual reconnaissance by a Greenland shark experienced in tracking live seals.
Gathering more data on the mysterious Greenland shark
To understand more the Greenland Shark and Elasmobranch Education and Research Group (GEERG) has been conducting non-lethal studies on the Greenland shark with the help of researchers from Canadian universities and institutions as well as seaside communities and fishers. GEERG has started a range of projects that focus on Greenland shark behaviour, species distribution, DNA analysis, telemetry, fisheries statistics, shark conservation and public perception of sharks. To track the sharks GEERG is using acoustic telemetry allowing scientists to remotely measure and report information on the sharks. Emitter tags are attached to individual sharks and they send an electronic signal at regular intervals. Underwater receives placed at strategic locations then record the data that includes depth, temperature and swimming speed. The receivers are deployed and recovered by dive teams when needed. The data collected then enables scientists to make statistical graphs, which allow researchers to make statistical graphs to better understand the effects of changes in conditions on the movement of the Greenland shark. In the long-run this will enable scientists to potentially predict when sharks may or may not be present under certain conditions and at specific times of the year to ensure they don’t get caught up in commercial fishing activities.